1. Field of the Invention
The present invention relates to authentication technology for use with an image obtained from multiple exposure.
2. Description of the Related Art
Digital image data captured with a digital camera is easy to edit using photo-retouching tools commercially available for personal computers, for example. In other words, this means that image data obtained with a digital camera is easy to alter. For this reason, images represented by digital image data are less reliable than conventional silver halide photographs and accordingly there is the problem that such digital images are less acceptable as video evidence. In order to address this problem, methods have been proposed as disclosed in U.S. Pat. No. 5,499,294 (hereinafter referred to as “Document 1”). According to Document 1, secret information unique to each digital camera is embedded in advance in the camera. When image data is obtained with the digital camera, the secret information is used to perform signature processing on the obtained image data inside the digital camera. By performing verification processing using resultant signature information after image capture, the fidelity of the captured image data to its original can be ensured.
Meanwhile, some digital cameras have a multiple exposure mode in which multiple exposure is performed on the same image data as disclosed in Japanese Patent Laid-Open No. 2006-174026 (hereinafter referred to as “Document 2”). For example, in the case of image data obtained from multiple exposure including two exposures, a piece of image data recorded with the first exposure and a piece of image data recorded with the second exposure are combined within a digital camera, and resultant composite image data is output (stored) as a final captured image from the digital camera to a nonvolatile memory. Such an image obtained in the multiple exposure mode is not a true image because that image is a composition of images captured at different times. In other words, an image obtained from multiple exposure can represent a scene that is not perceptible by humans.
If signature processing as disclosed in Document 1 is performed on image data obtained in the multiple exposure mode as disclosed in Document 2, the image data will successfully pass signature verification if it was not altered after image capture. This verification result demonstrates a new problem whereby a image data may be misconceived by the verifier as a real scene that did actually exist.